1. Field of the Invention
The present invention relates to an optical fiber composite insulator which is mainly used in the formation of a detection system for detecting fault points in electric power transmission networks, distribution systems, substations and the like, and a method of producing the optical fiber composite insulator.
2. Related Art Statement
Hitherto, it has been desired to develop a system for quickly detecting and repairing fault points caused by thunderbolts or the other in electric power transmission networks, distribution systems, substations and the like. Detection systems have been used including photosensors provided with Pockels and Faraday elements for detecting an extraordinary voltage and current.
In such a detection system, it is required to electrically insulate the photosensor from a fault point detector and a display by means of an optical fiber composite insulator.
Therefore, the optical fiber composite insulator used in the detection systems is required not only to effectively transmit optical signals from the photo-sensors to the detectors through at least one optical fiber but also to maintain the necessary mechanical strength and electrical insulating function nearly equal to those of the conventional solid insulator for a long useful life.
Various optical fiber composite insulators have been known. For example, Japanese Patent Application Laid-open Publication No. 60-158402 discloses a technique of hermetically sealing one or more optical fibers passed through a central through-hole in a hollow insulator body by wholly or partly filling an organic sealing material such as silicone rubber within the clearance between the optical fiber or fibers and the inner surface of the central through-hole to thereby prevent the decrease of the surface leakage-insulating distance, and also discloses a technique of sealing the optical fibers by wholly filling an inorganic sealing material such as a melted glass within the clearance in the central through-hole in the hollow insulator body after the entire insulator is previously heated.
Furthermore, there has been known a technique of hermetically sealing the optical fibers by partly filling an organic insulating material such as silicone rubber within only the axially intermediate portion of the clearance in the central through-hole and sealing the optical fibers within remaining clearances in the opposite end portions of the through-hole by melted glass.
The conventional optical fiber composite insulators hermetically sealed by using the organic material such as silicone rubber as a sealing material for maintaining the necessary airtightness between the optical fiber and the inner surface of the through-hole in the hollow insulator body can be easily produced since a heat treatment at high temperature is not required in an optical fiber sealing process after producing the insulator bodies. However, optical fiber composite insulators are usually exposed to severe conditions of temperature and humidity such that the surface temperature is raised to about 60.degree. C. by absorbing a heat from the sun in midsummer and is conversely lowered to about -20.degree. C. by radiational cooling in midwinter, and are continuously loaded with an electric power. Thus, the weather resistance of the the sealing material is too deteriorated to effectively use the optical fiber composite insulators in the detection systems if the sealing materials are not adequately selected. Also, adhesion interfaces between the silicone rubber or glass and the optical fibers and between the inner surface of the through-hole of the hollow insulator body and the silicone rubber or glass should keep the necessary airtightness even if the optical fiber composite insulator is exposed to a variation of circumferential temperature, humidity and the other for long term. However, a relationship between the inner surface of the through-hole and the organic or inorganic sealing materials for satisfactorily sealing the optical fiber under such a condition has not been considered.
Furthermore, the conventional optical fiber composite insulators comprising a hollow insulator body have a mechanical strength lower than that of the conventional solid insulators of the same outer diameter. Moreover, the optical fiber composite insulator comprises the hollow insulator body having the central through-hole filled with a cured material having a different coefficient of thermal expansion and, as a result, the mechanical strength is further decreased by a residual stress.
Also, the optical fiber composite insulator, must have the same mechanical strength as that of the conventional solid supporting insulator, since it is required to be interchangeable with the conventional solid supporting insulator usually used in disconnectors. However, the conventional optical fiber composite insulator has not been elucidated about a relationship between the inner diameter of the through-hole and the outer diameter of the barrel portion of the hollow insulator body and an effect of the sealing material filled in the through-hole on the mechanical strength of the optical fiber composite insulator.